Merge of attr_encryptor functionality back into attr_encrypted

This change merges the functionality of attr_encryptor (https://github.com/danpal/attr_encryptor) back into attr_encrypted and makes this enhanced encryption behaviour (":per_attribute_iv_and_salt") the default while retaining a compatibility mode (":single_iv_and_salt") for backwards compatibility with data encrypted with earlier versions of this gem.

Note that this update depends on the following pull request into the underlying Encryptor project (https://github.com/shuber/encryptor/pull/9) and will not function without it.